1. Technical Field
This disclosure relates to an alloy steel powder that can be used for powder metallurgy.
2. Description of the Related Art
Powder metallurgy technology allows components that require high dimensional accuracy and have a complex structure to be produced with near net shape, thereby significantly decreasing the finishing cost. Therefore, many products produced by powder metallurgy are used as various components for machines and apparatuses in many fields.
Recently, as components have been reduced in size and in weight, high rolling contact fatigue strength has been a strongly desired characteristic of iron-based powder metallurgy products.
In general, green compacts using an iron-based powder are produced as follows: An iron-based powder is mixed with powders for an alloy such as copper powder and graphite powder, and lubricant powders such as stearic acid and lithium stearate to prepare an iron-based mixed powder. This iron-based mixed powder is filled in a die and then subjected to compacting.
Iron-based powders are classified into, for example, iron powders (such as pure iron powders) and alloy steel powders depending on the component. Also, iron-based powders are classified into, for example, atomized iron powders and reduced iron powders depending on the method of production. In this case, “iron powders” also include alloy steel powders in a broad sense.
Green compacts produced by a general powder metallurgy process generally have a density of 6.6 to 7.1 Mg/cm3. Subsequently, these green compacts of an iron-based powder are sintered to form sintered bodies. The sintered bodies are subjected to a sizing or a cutting process according to needs. Thus, powder metallurgy products are produced. Furthermore, the products are subjected to heat treatment such as carburizing or bright-quenching after sintering when higher rolling contact fatigue strength is required.
Applying a high alloy is useful for improving, for example, the tensile strength of the powder metallurgy product. In such a case, however, an alloy steel powder, which is a raw material, is hardened, thereby decreasing compressibility. Unfortunately, the load of the equipment in compacting is increased. In addition, the decrease in compressibility of the alloy steel powder offsets the increase in the strength because the density of the sintered body is decreased. Accordingly, a technology for increasing the strength of the sintered body and suppressing the decrease in the compressibility is desired.
According to a general technology for increasing the strength of the sintered body while maintaining compressibility, alloying elements such as Ni, Cu, and Mo, which improve hardenability, are added to the iron-based powder.
For example, according to Japanese Examined Patent Application Publication No. 63-66362, molybdenum (Mo) is used as an effective element for the above purpose. In the above patent document, Mo is added to an iron powder as a prealloyed element so long as compressibility is not impaired (Mo: 0.1 to 1.0 mass percent). Copper powders and nickel powders are bonded on the surfaces of the iron particles by diffusion bonding. According to this technology, both preferable compressibility during the compacting and high strength of the components after the sintering are obtained.
Japanese Unexamined Patent Application Publication No. 61-130401 discloses an alloy steel powder for powder metallurgy to produce a sintered body having high strength. According to the above patent document, at least two alloying elements, in particular, Mo and Ni, or Mo, Ni, and Cu, are bonded on the surfaces of steel powders by diffusion bonding. According to this technology, the concentrations of the alloying elements bonded on the surfaces of the steel powders by diffusion bonding are controlled as follows: The concentration of each alloying element bonded on the surfaces of fine steel powders having a diameter of 44 μm or less is controlled to be 0.9 to 1.9 times the concentration of each alloying element bonded on the surfaces of all steel powders. This relatively wide range of limitation provides a preferable impact toughness to the sintered body.
In view of the recent issues regarding environmental protection and recycling efficiency, however, the use of Ni and Cu has disadvantages and should be avoided as much as possible.
A Mo-containing alloy steel powder which does not contain Ni or Cu and in which Mo is the main alloying element is also disclosed. For example, an alloy steel powder disclosed in Japanese Examined Patent Application Publication No. 6-89365 includes 1.5 to 20 mass percent of Mo, which is a ferrite-stabilizing element, as a prealloy. In such a case, sintering is accelerated by forming a single a phase in which the self-diffusion rate of Fe is high. The use of this alloy steel powder provides a sintered body having a high density because of the matching of a step of pressure sintering with, for example, particle size distribution. In addition, the use of this alloy steel powder provides a homogeneous and stable structure because this powder does not include an alloying element bonded by diffusion bonding. However, the Mo content in the disclosure is relatively high, namely at least 1.8 mass percent. Unfortunately, in this alloy steel powder, compressibility is low and, therefore, a green compact having a high density cannot be produced. Consequently, when the green compact is subjected to a general sintering step (i.e., sintering in one step without pressurizing), the sintered body has a low density.
Japanese Unexamined Patent Application Publication No. 2002-146403 also discloses an alloy steel powder for powder metallurgy containing Mo as a main alloying element. According to this technology, 0.2 to 10 mass percent of Mo is bonded on the surface of iron-based powder by diffusion bonding, the iron-based powder containing 1.0 mass percent or less of Mn, or further containing less than 0.2 mass percent of Mo as the prealloy. This alloy steel powder has superior compressibility and provides a sintered body having a high density and high strength. However, a process of powder metallurgy that includes repressing and resintering of the sintered body is applied to this alloy steel powder. Therefore, a general method for sintering does not sufficiently provide the above advantage.
Japanese Examined Patent Application Publication No. 7-51721 discloses a ferroalloy powder (alloy steel powder) wherein 0.2 to 1.5 mass percent of Mo and 0.05 to 0.25 mass percent of Mn are added to an iron powder as prealloyed elements. This ferroalloy powder is a low alloy and has a relatively high compressibility in compacting. Furthermore, this ferroalloy powder provides a sintered body having high strength.
According to the technologies described above, however, the alloys are not designed to consider rolling contact fatigue strength. As described above, recently, high rolling contact fatigue strength is strongly desired in sintered metal components. Such high rolling contact fatigue strength is difficult to achieve, even when the above alloy steel powders are sintered by a general sintering step.
For example, the following problem resides in the ferroalloy powder disclosed in Japanese Examined Patent Application Publication No. 7-51721. When the ferroalloy powder is sintered at a temperature (in general, 1,120° C. to 1,140° C.) of a mesh belt furnace, which is generally used for powder metallurgy, the sintered body does not have a sufficiently high rolling contact fatigue strength. The reason for this is that the progress of sintering between the particles is not sufficiently accelerated and, therefore, the reinforcement of a sintering neck (i.e., a part where the sintering reaction starts, which will be described later) is insufficient.
For example, Japanese Unexamined Patent Application Publication Nos. 6-81001 and 2003-147405 disclose technologies in view of rolling contact fatigue strength. According to the technology disclosed in Japanese Unexamined Patent Application Publication No. 2003-147405, 0.5 to 1.5 mass percent of Mo is bonded on the surfaces of a steel powder containing 0.5 to 2.5 mass percent of Ni and 0.3 to 2.5 mass percent of Mo as the prealloy by diffusion bonding. The sintered body after carburizing and quenching has a maximum fatigue strength of about 2.5 GPa, which is measured by a Mori-type rolling contact fatigue tester. However, recently, higher rolling contact fatigue strength has been desired.
Japanese Unexamined Patent Application Publication No. 6-81001 discloses the following alloy steel powder. An iron-based powder contains 0.05 to 2.5 mass percent of Mo and at least one element selected from the group consisting of V, Ti, and Nb as the prealloy. Nickel and/or copper is bonded on the surface of the above iron-based powder by diffusion bonding. According to that alloy steel powder, the sintered body after carburizing and quenching only has a maximum rolling contact fatigue strength of about 260 kgf/mm2 as measured by the Mori-type rolling contact fatigue tester.
Accordingly, in view of the above problems, it would be advantageous to provide an alloy steel powder for powder metallurgy that has high rolling contact fatigue strength even after sintering at a relatively low temperature, while maintaining high density of the sintered body (i.e., high compressibility of the alloy steel powder).